HERCULES -- The municipal swimming pool leaked millions of gallons of water into the ground over the past two years, but apparently no one noticed, or nothing was done about it, until a finance official inquired about water bills over the summer.

The approximately 14 million gallons that seeped from the pool cost the city an extra $75,000 on its water bills over that period, officials estimate. That sum does not factor in possible extra costs for heating and chemically treating the replacement water.

Parks and Recreation Director Pedro Jimenez said his department learned about a possible leak after assistant civil engineer Jose Pacheco verified water meters for the Finance Department.

"The information informed us of the spike in water usage over the past two years and that there was a good chance of a leak somewhere in the pool or in the system," Jimenez said.

A contractor fixed the leak earlier this month for $4,200, the cost of replacing a mastic joint, plus $600 to track down the leak last month.

Fortunately, the leaking water has not eroded the ground under the pool or otherwise threatened the pool's stability, according to Municipal Services Director John McGuire, who said he thoroughly reviewed the construction documents for the swim center and its two pools -- the main pool and a smaller wading pool -- and conducted a physical inspection of the site.

"There is no sign of failure anywhere," McGuire said. "The pool infrastructure was designed and built to drain groundwater around and below the pools."

A system of drains and pipes connected to sounding wells leads into an outfall pipe and slowly drains toward nearby Refugio Creek.

"The sounding wells and drainage pipes are dry, and there is nothing coming out of the outfall," McGuire said. "There is no sign that dirt discharged from the pipe or that there was a failure anywhere in the hillside as one would expect if there was a problem. In addition, the pool remains completely level."

If there were a settling problem, the pool would begin to list to one side and water would lap over the edge on the low side, but that has not happened, McGuire said.

Jimenez, the parks and recreation director, said that going forward, "We will be working with finance on tracking the water bills to see if there is any spikes."

Contact Tom Lochner at 510-262-2760. Follow him at Twitter.com/tomlochner.